1. Technical Field of the Invention
The present invention relates generally to the field of broadband communications and, more particularly, to a method and system of upstream power back-off within a broadband communication system.
2. Background of the Invention
It is becoming increasingly clear that telephone companies around the world are making decisions to include existing twisted-pair loops in their next generation broadband access networks. One attractive alternative, known as fiber-to-the-neighborhood (FTTN), is a combination of fiber cables feeding neighborhood Optical Network Unites (ONUs) and last leg premises connections by existing or new cooper. One of the enabling technologies for FTTN is very high rate Digital Subscriber Line (VDSL). VDSL transmits high speed data over short reaches of twisted-pair copper telephone lines, with a range of speeds depending upon actual line length and direction of transmission.
A VDSL requirement to use power back-off in the upstream direction arises because distributed topologies must be presumed for VDSL. In general, power back-off causes decreases in data rates relative to when all loops are the same length and only equal-length crosstalk results. However, it is difficult to project what the data rate loss will be with a particular power back-off algorithm. The data rates are a function of the loop topology and power back-off parameters.
Referring now to FIG. 1, there is illustrated a broadband access network with a plurality of varying length transmission lines in which the lines terminate at an ONU 10. Projections of downstream VDSL performance that assume self-FEXT are pessimistic when lines of varying lengths reside in the same binder as in FIG. 1. The assumption of self-FEXT is more accurate for short lines, on which FEXT levels are highest. For long lines, the assumption of self-FEXT is pessimistic because the shorter lines do not couple over the entire length of the long lines.
In the upstream direction, however, the assumption of self-FEXT is optimistic for longer lines. If all VTU-Rs transmit at their maximum power spectral density (PSD) levels, signals on shorter lines will detrimentally affect the upstream performance on longer lines. To illustrate, assume the maximum VDSL transmit PSD is −60 dBm/Hz. Referring to FIG. 1, the signal transmitted by the VTU-R on LN at this level will be attenuated significantly by the time it travels a distance (dN-dl). At this point, transmissions from the VTU-R on Ll may begin to couple into LN. If the transmit PSD of the VTU-R on Ll is −60 dBm/Hz, then it is significantly higher than the attenuated level of the desired signal on LN. The result of the relatively high-power interference is a degradation in achievable upstream rate on LN.
Simulations to project VDSL performance generally assume that any far-end crosstalk at the VDSL receiver is due to disturbers that are identical to the line under consideration. In other words, it is assumed that all disturbers span the same distance and transmit the same power spectrum as the line under consideration, and “self-FEXT” results. In reality, lines emanating from a single ONU, CO or LEx may span a variety of distances in a distributed topology, as shown by the example in FIG. 1.
The near-far problem is most severe when a very long line is degraded by FEXT caused by shorter lines on which “too high” an upstream transmit PSD is used. Loosely speaking, the transmit PSD on a short line is “too high” if its level is significantly higher than the level of the signal on the long line at the point at which the shorter line begins to couple into the longer line.